The use of electrical measurements in prior art downhole applications, such as logging while drilling (LWD) and wireline logging applications, is well known. Such techniques may be utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, may be used to indicate the presence of hydrocarbons in the formation. For example, it is known in the art that porous formations having a high electrical resistivity often contain hydrocarbons, such as crude oil, while porous formations having a low electrical resistivity are often water saturated. It will be appreciated that the terms resistivity and conductivity are often used interchangeably in the art. Those of ordinary skill in the art will readily recognize that these quantities are reciprocals and that one may be converted to the other via simple mathematical calculations. Mention of one or the other herein is for convenience of description, and is not intended in a limiting sense.
Techniques for making microresistivity measurements of a subterranean formation are well known in the prior art for both wireline and LWD operations. For example, microresistivity sensors configured for use with non-conductive drilling fluid commonly include at least one pair of potential electrodes deployed between a current injection electrode and a corresponding current return electrode. In use, alternating current is passed between the injector and return electrodes and the potential difference (voltage drop) between the potential electrodes is measured. The formation resistivity in the region of the potential electrodes may then be calculated from the measured potential difference. Those of ordinary skill will appreciate that the formation resistivity tends to be approximately proportional to the measured potential difference.
Techniques for making microresistivity anisotropy measurements are also known in the art. For example, U.S. Pat. No. 6,765,386 to Gianzero et al discloses a logging tool including first and second current injection electrodes. The first current injection electrode is vertically spaced apart from a corresponding return electrode and is configured to induce a vertical current. The second current injection electrode is horizontally spaced apart from a corresponding return electrode and is configured to induce a horizontal current. The tool further includes multiple pairs of potential electrodes deployed between the current injection and return electrodes. In use, current is first applied between the first pair of current electrodes and then later applied between the second pair of current electrodes to obtain microresistivity measurements.
While such measurement techniques may be suitable for making microresistivity anisotropy measurements, there is yet room for further improvement, in particular for making azimuthally resolved microresistivity anisotropy measurements during LWD imaging applications.